Conventionally, an IF (Intermediate Frequency) signal has been generated by an orthogonal modulation. FIG. 6 shows an orthogonal modulation circuit according to prior art.
With reference to FIG. 6, base band signals include an I signal and a Q signal. The I signal is amplified by an amplifier 102. A multiplier 104 mixes the amplified signal with a local signal generated by a local signal source 300. The Q signal is amplified by an amplifier 202. The phase of the local signal generated by the local signal source 300 is shifted by a phase shifter 304 by 90 degrees. Then, the multiplier 204 mixes the Q signal amplified by the amplifier 202 and the local signal whose phase has been shifted by 90 degrees with each other. An adder 400 adds an output from the multiplier 104 to an output from the multiplier 204, and outputs an added signal as an IF signal.
On this occasion, it is difficult to precisely maintain a phase difference between the local signals supplied respectively to the multiplier 104 and the multiplier 204 to 90 degrees. A phase error is thus generated. Moreover, since the base band signals include two systems: the I signal and Q signal, there may be difference in the amplitude between the I signal and the Q signal. Amplitude errors are thus generated. Therefore, it is necessary to remove these errors, namely to carry out a calibration.
For the calibration, signals used for the calibration are supplied as the I signal and Q signal. The calibration is carried out based upon a signal output from the adder 400 as a result of supplying the calibration signals.
Note that Japanese Laid-Open Patent Publication (Kokai) No. 2001-333120 describes a calibration for a demodulator.
However, during the calibration, an IF signal cannot be generated by means of the orthogonal modulation.
The present invention has an object of providing an orthogonal modulation device and the like which can carry out a calibration without stopping a modulation carried out by the orthogonal modulation device.